The present invention relates to an improved process for preparing N,N-diacetic acid aminomethylenephosphonic acid also known chemically as N-phosphonomethyliminodiacetic acid of the formula (I): ##STR1## The above compound (I) is an intermediate in the preparation of N-phosphonomethylglycine (glyphosate), an important broad spectrum herbicide. More particularly, the present invention relates to an improved chemical route to (I) in which iminodiacetonitrile (IDAN) is the starting material. IDAN has previously been converted to iminodiacetic acid (IDA) by various process steps beginning with hydrolysis of IDAN with an alkali metal hydoxide, usually sodium hydroxide. This process is described in U.S. Pat. No. 3,904,668.
The practice heretofore in the preparation of IDA for utilization in a phosphonomethylation process to produce N,N-diacetic acid aminomethylenephosphonic acid was to recover IDA from the crude hydrolysate of IDAN by (1) acidification with a mineral acid (typically sulfuric or hydrochloric acid), (2) crystallization of IDA, (3) filtration to recover the crystallized IDA, and (4) drying the IDA for packaging, shipping, etc. A similar recovery of IDA is taught in British Pat. No. 1,575,469. Also, the sodium salt solution separated from IDA in (3) above contained unrecovered IDA which was recovered by evaporating water from the solution resulting in precipitation of sodium salt while leaving the IDA in solution. The precipitated sodium salt was then separated from the residue by filtration and the filtrate recycled to Step (1) above. The above-described process is energy intensive and requires a large investment for the acquisition and maintenance of equipment to recover and purify IDA.
In the past, the recovered IDA from (4) above was utilized in a phosphonomethylation process such as that disclosed in U.S. Pat. No. 3,288,846 to Irani et al, particularly Example IV. In such process the hydrochloride salt is initially formed which is then phosphonomethylated with phosphorous acid (H.sub.3 PO.sub.3) and formaldehyde (CH.sub.2 O). In an alternate method the hydrogen chloride employed to form the hydrochloride salt of IDA and the phosphorous acid employed in phosphonomethylation are provided by the addition of phosphorus trichloride to water. In water, phosphorus trichloride is hydrolyzed to form hydrogen chloride and phosphorous acid. After phosphonomethylation the desired N,N-diacetic acid aminomethylenephosphonic acid is recovered from the reaction mixture by crystallization and filtration. Under current practice there is sufficient unreacted material in the filtrate to require recycle of the filtrate. A large amount of hydrogen chloride is released during the hydrolysis of phosphorus trichloride, if employed, and is recovered.
Although the above-described procedures are commercially feasible, the need for a reduction in the amount of energy consumed and equipment required makes further improvement highly desirable.